Heterogeneous polymer blends comprising an elastomeric copolymer dispersed in a continuous phase of a polypropylene are well-known and, depending on the properties and the relative amounts of the first and elastomeric copolymers, a wide variety of such polymer blends can be produced. Of particular interest are polymer blends, also referred to as thermoplastic elastomers, in which the polypropylene is a thermoplastic material, such as polypropylene, and the elastomeric copolymer is an elastomeric material, such as an ethylene-propylene elastomer or an ethylene-propylene-diene (EPDM) rubber. Examples of such thermoplastic elastomers include polypropylene impact copolymers, thermoplastic olefins and thermoplastic vulcanizates.
Unlike conventional vulcanized rubbers, thermoplastic elastomers can be processed and recycled like thermoplastic materials, yet have properties and performance similar to that of vulcanized rubber at service temperatures. For this reason, thermoplastic elastomers are useful for making a variety of articles such as weather seals, hoses, belts, gaskets, moldings, boots, elastic fibers and like articles. They are also particularly useful for making articles by blow molding, extrusion, injection molding, thermo-forming, elasto-welding and compression molding techniques. In addition, thermoplastic elastomers are often used for making vehicle parts, such as but not limited to, weather seals, brake parts including, but not limited to cups, coupling disks, diaphragm cups, boots such as constant velocity joints and rack and pinion joints, tubing, sealing gaskets, parts of hydraulically or pneumatically operated apparatus, o-rings, pistons, valves, valve seats, and valve guides.
One method of making the aforementioned polymer blends is by mixing two different polymers after they have been polymerized to achieve a target set of properties. However, this method is relatively expensive making it much more desirable to make blends by direct polymerization. Blending by direct polymerization is well known in the prior art and typically uses multiple reactors in series, where the product from one reactor is fed to a second reactor having a different polymerizing environment, resulting in a final product that is an intimate mix of two different products. Examples of such processes employing vanadium catalysts in series reactor operation to produce different types of EPDM compositions are disclosed in U.S. Pat. Nos. 3,629,212, 4,016,342, and 4,306,041.
U.S. Pat. No. 6,245,856 discloses a thermoplastic olefin composition comprising polypropylene, an ethylene-α-olefin elastomer and a compatabilizer comprising an ethylene-propylene copolymer having a propylene content of greater than 80 wt %. According to this patent, the individual components of the composition can be separately manufactured and mechanically blended together in a mechanical mixer or two or more of the components can be prepared as a reactor blend using a series of reactors where each component is prepared in a separate reactor and the reactant is then transferred to another reactor where a second component is prepared. In the absence of the compatabilizer, the elastomer phase is said to be uneven with particle size greater than 5 microns, whereas the addition of the compatabilizer is said to improve dispersion such that the elastomer phase has a particle size of about 1 micron. The elastomer phase of this polymer blend is not cross-linked.
U.S. Pat. No. 6,207,756 describes a process for producing a blend of a continuous phase of a semi-crystalline plastic, such as polypropylene, and a discontinuous phase of an amorphous elastomer, such as a terpolymer of ethylene, a C3-C20 α-olefin and a non-conjugated diene. The blends are produced in series reactors by producing a polypropylene component in a first reactor, directing the effluent to a second reactor and producing the elastomeric copolymer component in solution in the second reactor in the presence of the polypropylene component. U.S. Pat. No. 6,319,998 also discloses using series solution polymerizations to produce blends of ethylene copolymers. U.S. Pat. No. 6,770,714 discloses the use of parallel polymerizations to produce different polymeric components that are then blended through extrusion or using other conventional mixing equipment. One polymeric component is a propylene homopolymer or copolymer and the elastomeric copolymer component is an ethylene copolymer.
One particularly useful form of thermoplastic elastomer is a thermoplastic vulcanizate (“TPV”), which comprises a thermoplastic resin continuous phase, such as polypropylene, within which are dispersed particles of a vulcanized elastomeric material, such as an EPDM rubber. TPVs are normally produced by a process of “dynamic vulcanization”, which is a process of vulcanizing or cross-linking the elastomeric component during intimate melt mixing with the thermoplastic resin, together with plasticizers (e.g. process oils), fillers, stabilizers, and a cross-linking system, under high shear and above the melting point of the thermoplastic. The mixing is typically done in a twin-screw extruder, to create a fine dispersion of the elastomeric material within the thermoplastic resin while the elastomeric material is vulcanized. The levels of thermoplastic resin and plasticizer (oil) can be adjusted to produce grades having different profiles of hardness, rheology and engineering properties, although in general it is difficult to produce TPVs by dynamic vulcanization in which the content of the elastomeric phase is greater than 50 wt % of the overall polymer blend. Examples of dynamic vulcanization are described in the U.S. Pat. Nos. 4,130,535 and 4,311,628.
However, while dynamic vulcanization is effective in producing TPVs with a unique profile of properties, it is expensive and suffers from a number of disadvantages. Thus the production of quality product is technically challenging and specialized equipment is needed. Moreover, the process involves many steps, each one critical to the eventual quality of the final product. Forming the polymer blend normally involves separately comminuting bales of the elastomeric polymer (which is typically how EPDM rubber is commercially distributed), mechanically mixing it with the thermoplastic resin along with the processing oils, cross-linking agents, and other ingredients in a suitable high shear mixing device to comminute the rubber particles and vulcanize them to generate vulcanized rubber particles embedded in a continuous thermoplastic resin continuous phase. The vulcanized rubber particles in the finished products have an averaged particle size of 1 to 10 micron. While the products produced with existing technology have many desirable properties, there are gaps in the overall properties profile. Some of these are the need for higher service temperatures, improved elastic recovery, softer products, higher Tensile Strength, easier processability, oil-free compositions, and colorless products.
An improved process for producing TPVs is disclosed in U.S. Pat. No. 6,388,016, incorporated herein in its entirety, in which a polymer blend is produced by solution polymerization in series reactors employing metallocene catalysts and the resultant blend is subjected to dynamic vulcanization. It will, however, be seen that this improved process still relies on dynamic vulcanization to vulcanize the elastomeric component. As a result the vulcanized diene-containing particles have an average particle size in the range of 1 to 10 microns.
An in-reactor process for producing cross-linked polymer blends, such as TPVs, is disclosed in our co-pending US 2006/0293455 filed Jun. 16, 2006. In this process, at least one first monomer is polymerized to produce a thermoplastic polypropylene; and then at least part of the polypropylene is contacted with at least one second monomer and at least one diene under conditions sufficient to produce and simultaneously cross-link an elastomeric copolymer as a dispersed phase within a continuous phase of the polypropylene. In the resultant polymer blend, the thermoplastic polypropylene has a crystallinity of at least 30% and the dispersed phase comprises particles of the elastomeric copolymer having an average size of less than 1 micron, wherein the elastomeric copolymer has a crystallinity of less than 20% and is at least partially cross-linked. In this way, the need for a separate dynamic vulcanization step to cross-link the elastomeric copolymer is avoided. However, for certain applications, it is desirable to enhance the level of curing of the elastomeric copolymer beyond that achieved by the in-reactor cross-linking. Accordingly, the present invention seeks to provide a polymer blend, and a process of its production, having an enhanced level of curing.